A device (104) for controlling a clock signal generator includes a processing system configured to form a control quantity at least partly on the basis of reception moments of data frames belonging to a given flow, the reception moments being time values based on a clock signal prevailing at a receiver. The processing system controls the clock signal generator with the control quantity so as to achieve synchronization between the clock signal and another clock signal in accordance of which the data frames have been transmitted. In order to identify data frames belonging to the relevant flow, the processing system checks whether pre-determined bits of a received data frame constitute a bit pattern that occurs when the data frame belongs to the relevant flow. Therefore, the reception moments of data frames belonging to the relevant flow are obtainable without deep inspection of the received data frames.